The present invention relates to a stepping projection aligner (hereinafter referred to as "stepper") which is designed to form a fine-line pattern of an integrated circuit (IC), a large-scale integrated circuit (LSI), etc., on a semiconductor substrate by exposure. More particularly, the present invention relates to a projection lens system which is useful to form by exposure an integrated circuit pattern on a semiconductor substrate by using a light source which emits light in the wavelength range of from the ultraviolet region to the vacuum ultraviolet region, i.e., of the order of 300 nm to 150 nm, for example, an excimer laser.
Hitherto, a stepper has been used to form a pattern of an IC, LSI, liquid crystal display, thin-film magnetic head, etc., on a semiconductor or other substrate by exposure. With the recent achievement of high integration density of integrated circuits, the projection lens system of the stepper has also been demanded to have a higher resolving power and a wider exposure area.
In general, the following relations hold between the resolving power and depth of focus of a lens system, the wavelength and the numerical aperture : EQU Resolving power=k.sub.1 .multidot..lambda./NA EQU Depth of focus=k.sub.2 .multidot..lambda./NA.sup.2
where .lambda. is the wavelength; NA is the numerical aperture and k.sub.1 and k.sub.2 are proportional constants depending upon the process.
Accordingly, it is necessary in order to improve the resolving power of the projection lens system to shorten the exposure wavelength used or to increase the NA. However, the scheme of increasing the NA invites a rapid reduction of the depth of focus and also involves difficulty in optical design.
Under these circumstances, it has been proposed to achieve high resolving power by using a shorter exposure wavelength. More specifically, since it has become impossible to obtain resolving power sufficient for the present demand with the g-line (436 nm) and i-line (365 nm) of a super-high pressure mercury lamp used in the conventional steppers, KrF excimer laser (248 nm) and ArF excimer laser (193 nm) are considered to be promising as being next generation light sources.
On the other hand, the conventional practice for widening the exposure area of a projection lens system is to increase the object-image distance of the projection lens system (i.e., to multiply the projection optical system by a coefficient) or to use an aspherical lens. The former practice intends to enlarge the exposure area by multiplying the projection optical system by a coefficient because there is a limit to the achievement of a wider exposure area as long as spherical quartz lenses are used as in the conventional practice.
Incidentally, when an excimer laser is used as a light source in order to improve the resolving power, some problems arise: That is, since the half-width of excimer laser light is as large as 0.3 nm to 0.4 nm in a free run, the projection lens system needs achromatism. However, in the wavelength region of excimer laser light, the transmittance of ordinary glass is insufficient, and hence usable vitreous materials are limited to quartz, fluorite, MgF.sub.2, etc. Fluorite is low in hardness and hence damageable and cannot readily be subjected to optical polishing. MgF.sub.2 is deliquescent and anisotropic. Thus, vitreous materials other than quartz suffer from problems in terms of processability. Accordingly, a vitreous material that is practically usable for a projection lens system is limited to quartz.
With the above-described background, it is a common practice to form a projection lens system using monochromatic design lens elements made only of quartz and to narrow the wavelength spectral bandwidth of the light source, thereby preventing generation of chromatic aberration in the optical system. However, the optical system having such an arrangement suffers from the following problems:
1 As the result of narrowing the wavelength spectral band, the laser output lowers. PA1 2 The laser becomes complicated to maintain the center wavelength, the spectral half-width, etc. with high accuracy. PA1 3 Since the spectral half-width allowed for the light source is inversely proportional to NA.sup.2, the allowable half-width becomes extremely small as the NA of the projection lens system is increased with the achievement of high integration density of devices. PA1 where h is the effective aperture radius of the DOE, and p is the distance from the optical axis to the no-power portion of the DOE. PA1 where h is the effective aperture radius of the DOE, and t is the height of the most off-axis chief ray in the DOE.
The scheme of widening the exposure area of the projection lens system involves the following problems: If the object-image distance is increased by coefficient-multiplying the projection lens system, the projection optical system becomes exceedingly large in size, and the stepper eventually becomes incapable of being accommodated in a conventional clean room; otherwise a gigantic clean room is needed.
Further, since the spectral half-width required for the light source is inversely proportional to the focal length of the projection lens, the scheme of coefficient-multiplying the optical system unfavorably necessitates further narrowing the spectral band.
In the case of refracting lenses, spherical and aspherical lenses are produced by different methods, and it is not easy to produce a highly accurate aspherical lens as is required for a stepper lens.
To solve these problems, a projection lens system that uses a diffractive optical element has been proposed, as is disclosed in Japanese Patent Application Post-Exam Publication No. JP-A-4-214516 (1992) (corresponding to U.S. Pat. Nos. 4,936,665, 5,156,943 and 5,386,319.
However, if the diffractive optical element is provided with only a positive power action for mainly correcting chromatic aberration, the pitch at the most peripheral portion of the effective aperture radius region of the diffractive optical element becomes exceedingly small, so that the production of the diffractive optical element becomes extremely difficult. Moreover, the effect of correcting aberrations other than chromatic aberration is not satisfactorily large.
Thus, many problems are attendant on the conventional projection optical system with a high resolving power and a wide exposure area that uses an excimer laser as a light source.